Objects. Theoretical Part. Contents. Keywords. Programming with Java Module 5. 1 Module Overview 3

Transcription

1 Programming with Java Module 5 Objects Theoretical Part Contents 1 Module Overview 3 2 Classes and Objects Classes Object variables and methods Object variables (attribute) Object methods Generating objects by using a class Difference between primitive data types and reference types 6 Keywords Classes Objects Object Properties Attribute Object Method Reference Type

2 Authors: Lukas Fässler, Barbara Scheuner, David Sichau Date: 18 July 2018 Version: 1.1 Hash: d2eb005 The authors try the best to provide an error free work, however there still might be some errors. The Authors are thankful for your feedback and suggestions. This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License. To view a copy of this license, visit 2

3 1 Module Overview The aim of the object-oriented programming is - analog to the real world - to represent data with their associated functionalities in the form of objects. Whilst in procedural programming data and the operations performed on them are separated from each other, they are combined into a single structure, the class, in object-oriented programming. A class is a description ( template or construction plan ) for objects that are characterized by identical or similar properties and behaviors. We can generate several objects (copies or instances) of a class. 2 Classes and Objects Objects serve to manage data (variables) and their associated functionalities (methods). We specify in the class which variables and methods the object consists of. You can imagine the variables of a class as elements of a list, as for example the following: Name Firstname Year of birth Occupation p1 Smith Joe 1942 Bricklayer p2 Brown Emily 1954 Electrician p3 Doe John 1991 Student The class (analog to the table caption) states that the variables name, firstname, yearofbirth and occupation shall be stored in the objects (here the individual rows). The corresponding objects p1, p2 and p3 appear as follows: p1: Person p2:person p3:person name: Smith name: Brown name: Doe firstname: Joe firstname: Emily firstname: John yearofbirth: 1942 yearofbirth: 1954 yearofbirth: 1991 occupation: Bricklayer occupation: Electrician occupation: Student Object methods work with this information. We can, for example, read or update a person s information via a method. It is essential to understand that the methods exist for each object, and therefore, they work with the variable values (attributes) of the corresponding object. 3

4 2.1 Classes A class contains the rules specifying what belongs to an object. In our case: name, first name, year of birth and occupation. A class is defined by the command class. Notation: public class Person { // Attributes of the object (object variables) // Functionality of the object (Object method) 2.2 Object variables and methods Object variables and methods define all elements which occur in each object. These are not declared static and thus they are tied to an object and not to the class. The object methods, usually simply referred to as methods, work with these variables. Methods are mainly used to change variables, announce their value or calculate an attribute Object variables (attribute) The attributes are properties which are stored in an object. Therefore, the state of an object can be stored in these variables. Our class Person that contains the rule for a person object looks as follows: public class Person { String name; String firstname; int yearofbirth; String occupation Object methods Classes are not only able to store data in form of variables but also provide functionalities. This functionality enables processing of data. For the class Person, for example, a method can be provided that outputs the information to a person (print). A further method returns the age of the person if the current year is passed (ageoutput): 4

5 public class Person { String name; String firstname; int yearofbirth; String occupation; void print(){ System.out.println(name +"/"+ firstname); System.out.println(yearOfBirth +"/"+ occupation); int ageoutput(int year){ int age= year-yearofbirth; System.out.println(alter); Not only parameters that are passed to the method but also the object variables (attributes) are available to an object method. 2.3 Generating objects by using a class To use a Person, an object of this type has to be generated first. For example the object p1 of type Person is declared and initialized as follows: 1. Declaration: Setting reference to the class Person and naming it p1: Person p1; In contrast to primitive types, the standard value is null instead of Initialization: Generating an instance of the class Person by using new and storing it in the reference p1: p1 = new Person(); These two steps are often written in one command: Person p1 = new Person(); The initialization of objects is usually done in a further class which wants to store its data in the offered class Person. The class HRmanagment shall be able to generate Person-objects. If this HRmanagment is the starting point of our program, it could look as follows: 5

6 public class HRmanagment { public static void main(string[] args){ //Declaration Person p1; //Generation of a new object p1 = new Person(); //Assignment of values for the attributes p1.name = "Smith"; p1.firstname = "John"; p1.yearofbirth = 1960; p1.occupation = "Teacher"; Difference between primitive data types and reference types If a variable shall manage an object, it is called a reference variable. There is no difference between a reference type and a data type in the declaration (int, char, double, etc.). This is in contrast to the initialization: Reference types hold a reference to objects and provide possibilities for accessing objects which are stored somewhere in the memory - similar to a link. The actual storing locations are irrelevant for programming. Example: Suppose we want to generate a new person p2 in our example (this would correspond to inserting a new row in the table). For this purpose, copying the reference is not sufficient. The following example demonstrates why we first have to generate a new object by using the new-operator before we can set the values of the variable anew. Given: int value = 8; Person p1 = new Person(); The current memory corresponds to the following figure 1: There is an 8 belonging to the variable name value and a reference to a Personobject accessible via the variable name p1 in the storing location of the primitive data type and the reference type, respectively. We can see that the year is the only storing location that contains a value. This is the case because the String also refers to a class (note the uppercase). If it has not been determined yet to which storing location a reference variable refers, null is initialized. We can assign attribute values as follows: 6

7 Figure 1: Memory after generating a new object. p1.name = "Smith"; p1.firstname = "John"; p1.yearofbirth = 1960; p1.occupation = "Teacher"; The memory changes as illustrated in figure 2. Figure 2: Memory after the assignment of the attribute values. We execute the following lines now: int value2 = value; Person p2 = p1; The memory appears itself as illustrated in figure 3. Figure 3: Memory after the initialization of p2 with p1. 7

8 For primitive data types, a new storing location is provided and the value is copied. This is in contrast to the reference variable: only the reference is copied but not the whole object. If the following lines are executed, the memory looks like in figure 4. value2 = 10; p2.yearofbirth = 1967; Figure 4: Memory after changing the attribut p2. The following table summarizes the differences between the reference type and primitive types. 8

9 Primitive Data types Reference types Number Includes the types Boolean, char, byte, short, long, float und double Default value 0 null Storing location Assignment to another variable Passing to a method The storing location stores the actual data that the primitive type contains. If the value of a primitive data type is assigned to another variable, a copy is generated. If a primitive type is passed to a method, only a copy of the primitive type is passed. The invoked method has no access to the original primitive value and is not able to modify it. However, the invoked method can modify the value copied. The number of reference type is unlimited since they are defined by the user. The storing location stores a reference to the data. If a reference type is assigned to another reference type, both point to the same object. If an object is passed to a method, the invoked method is able to modify the content of the object but not its address. 9